Relationship between environmental factors, age of onset and familial history in Japanese patients with psoriasis

Psoriasis is a chronic inflammatory disease that often involves the skin and joints. Psoriasis develops at any age and the distribution of age of onset of psoriasis is bimodal in Japan. Also, male predominance is distinct in Japanese psoriatic patients. To clarify the relationship between sex difference and habitual/environmental status, age and incidence of familial psoriasis, we analyzed data from the Fukuoka University Psoriasis Registry. A total of 1120 Japanese patients (751 men, 369 women) were analyzed. The male/female ratio was 2.03:1. Smoking and drinking habit, known as risk factors of psoriasis, were significantly more prevalent in men. Age-specific psoriasis-onset rate standardized by population showed bimodal distribution in both men and women; the younger peak was in their 30s for men and 10s in women; the second peak was in the 50s for both sexes. A familial history of psoriasis was seen in 6.3% of patients overall; however, female patients showed a significantly higher rate (8.7%) compared with men (5.1%, P = 0.024). When stratified by age of onset, the frequency of familial history was much higher among women with onset at less than 30 years (15.4%), compared with 30 years or more (5.3%, P = 0.0026). Our data suggest that genetic factors have a stronger influence in young women who experience fewer environmental factors such as smoking and drinking. This is the first study to show that there is a difference in the incidence of familial psoriasis depending on age of onset of psoriasis in Japan.